Well, no, actually, it isn't, but I did manage to save my dress. I cut three inches off either side of the body panels (and oh, by the way, ripping 240 inches of seam? NOT FUN.), sewed the sleeve assemblies back on, and it actually fits really well! The skirt feels a bit skimpy, but it's an underdress, so it won't be seen that much. At the current measurements, I think I can get this dress out of 2.5 yards of 60" fabric! Go me! I should go buy 2.5 yards of 60" muslin, just to test. And I really need a full-length mirror.
For now, though, it gets hung up to relax, after which I can hem it. On to the apron!